<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Title</title>
    </head>

    <style>

        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        a {
            text-decoration: none;
        }

        li {
            list-style: none;
        }

        .header {
            border-top: 1px solid #999999;
            border-bottom: 1px solid #999999;
        }

        ul {
            display: flex;
            width: 1200px;
            margin: 0 auto;
        }

        ul li {
            width: 20%;
            border-right: 1px solid #999999;
        }

        .header ul li a {
            padding-top: 10px;
            padding-bottom: 10px;
            font-size: 16px;
            color: black;
            display: block;
            text-align: center;
        }

        .header ul li:last-child {
            border-right: none;
        }

        .header ul li:nth-child(4) a {
            position: relative;
        }

        .header ul li:nth-child(4):hover a img {
            display: block;
        }

        .header ul li:nth-child(4) a img {
            position: absolute;
            top: 100%;
            border-left: 1px solid #999999;
            border-right: 1px solid #999999;
            border-bottom: 1px solid #999999;
            padding: 3px;
            left: 50%;
            display: none;
            transform: translateX(-50%);
        }

    </style>

    <body>


        <div class="header">

            <ul>
                <li><a href="">我要投资</a></li>
                <li><a href="">平台介绍</a></li>
                <li><a href="">新手专区</a></li>
                <li>
                    <a href="">手机微金所
                        <img src="images/code.jpg">
                    </a>
                </li>
                <li><a href="">个人中心</a></li>
            </ul>

        </div>

    </body>
</html>